http://news.bbc.co.uk/low/english/sci/tech/newsid_255000/255725.stm

The scientists say the common assumption that our ancestors in the genus
Homo developed their large brains after they began
eating the nutrient- and energy-rich animal foods necessary to fuel the
larger brains may now need reappraisal.

"Our results raise the possibility... that dietary quality improved [through
the consumption of animal foods] before the development
of Homo and stone tools about 2.5 million years ago," writes Matt Sponheimer
- also of Rutgers University in New Brunswick,
USA - in the journal Science.